1. Field of the Invention
This invention is directed toward apparatus and methods for delivering a flow of liquid at a constant, controllable flow rate over long periods of time, and more specifically toward a dual diaphragm permeation tube which provides a flow of gas at a controllable, constant volume flow rate over a wide range of ambient temperature variations for periods of one year or longer.
2. Background of the Art
Virtually all analytical measurement instrumentation requires initial and periodic calibration. Such instrumentation includes hydrocarbon "sniffers" which measure concentrations of hydrocarbons in fluids. This type of instrumentation also includes tracer systems with which small quantities of radioactive or non radioactive material are used to label or "tag" material of interest, and the concentrations of tagging material are subsequently measured to obtain the desired concentrations of tagged materials of interest. Gas chromatographs are routinely calibrated with gases of known concentrations at known, controllable flow rates. Other types and classes of analytical instrumentation requiring calibration using controlled flows of fluids, and especially controlled flows of gas, are well known to those skilled in the art of analytical measurements.
Permeation tubes containing a single permeable barrier have been used in the prior art to provide a flow of fluid for calibration and other purposes. These tubes are typically cylindrical with a solid wall from one end and an output at the opposite end. A permeable barrier is positioned within the tube in the solid cylinder and the output end. Fluid is stored in the tube at a predetermined initial pressure. By selecting the type and the dimension of the permeable barrier, fluid passing through the barrier and flowing from the output of the tube is thereby controlled. Stated another way, for a given storage pressure and at a given ambient temperature, the volume flow rate from the permeation tube is controlled by the type and geometry of the permeable barrier within the tube.
Materials used to form the permeable barrier depends upon the type or class of fluid for which flow is being controlled. Barrier materials may be elastomeric, although other materials may be used. Permeation tubes can be fabricated to control the flow of a wide range of chemicals including alcohol, ketones, acids, aldehydes, paraffins, liquefiable gases, aromatics, water, dimethyl sulfide and dimethyl disulfides.
As mentioned previously, many analytical instruments need to be calibrated with a calibration or "standard" fluid at a predetermined or "standard" flow rate. Since periodic calibration is usually required, it is important that a source of calibration gas provide flow at a controllable, constant volume flow rate over long periods of time such as one year or more. Ambient temperature of the calibration gas source can vary significantly over such long periods of time. Ambient temperature changes adversely affect the flow rate from prior art single barrier permeation tubes. Variations in ambient temperature induce changes in temperature of stored fluid within the solid cylindrical wall device and behind it the permeable barrier. If it is assumed that fluid is initially stored in the permeation tube at a constant pressure, ambient temperature changes therefore induce pressure changes in the fluid captured in the tube shell or housing and the barrier. These pressure changes, in turn, cause variations in volume flow rate through the barrier and through the tube output since the barrier is "passive". The required constant volume flow rate output for instrument calibration is, therefore, not provided by a single barrier permeation tube when the tube is exposed to varying ambient temperature.
In view of the above discussion of the prior art, an object of the present invention is to provide a permeation tube which provides a controllable output flow of fluid which is independent of variations in the ambient temperature of the tube.
Another object of the invention is to provide a permeation tube which provides a constant, controllable flow of output gas over long periods of time such as one year or longer.
Yet another object of the present invention is to provide a permeation tube which can provide controlled volume flow rate, independent of changes of ambient temperature, over a range of volume flow rates which are not obtainable with a single barrier device.
Another object of the invention is to provide a permeation tube which is rugged, relatively inexpensive to manufacture, initially fill and later refill, and suited for adverse field applications.
Still another object of the present invention is to provide a permeation tube which can easily be refilled with standard or "calibration" fluid to extend tube life.
There are other objects and applications of the present invention which will become apparent in the following disclosure.